How strict is Keck's science LOR requirement?

[andrewRA4]

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Hi all! For keck, they say 3 max LORs. Now I have 2 hard science recs, one of which I'm using and the other I would rather use in place of my research PI's. My 3rd slot is already occupied by another rec. They said they want a science professor, so I wanted to know if my research PI's rec will work? I did take independent research under him, which was classified under the CHEM department.